Impact of combined horizontal-vertical seismic excitation on R/C columns designed against horizontal excitation only

The present paper deals with the assessment of the structural behavior of conventionally designed reinforced concrete columns subjected to combined horizontal-vertical seismic excitation. The term "conventionally designed" designates structural members designed against the horizontal seismic component only. Apart from estimating critical response quantities, the paper identifies which specific design criteria are affected and which code requirements could be violated due to the vertical excitation action. For this purpose, a common reinforced concrete frame with four stories is designed in accordance with Eurocodes which permit to omit the vertical component. Subsequently, the seismic response of the columns for combined horizontal-vertical seismic excitation is evaluated by means of time-history analysis applying three recorded accelerograms. The whole investigation indicates that conventional columns detailed according to the current code provisions might violate fundamental design criteria in case of seismic excitation along horizontal and vertical directions concurrently. The main cause of this phenomenon is the normalized axial force fluctuation.